Fuel injector assemblies are often utilized in internal combustion engines for delivering a metered measure of fuel to the combustion chamber. The injected fuel mixes with air within the chamber for combustion. Precise metering and timing of the fuel injection provides for better combustion, thus better horsepower, fuel economy and performance. In addition precise control of the fuel injection has also been known to improve emission performance.
Piezo-electric material provides for a rapid response when a voltage is applied to the material. When piezo-electric material is incorporated in a control actuator of a Diesel fuel injector it is faster and more responsive than solenoid actuated or mechanically actuated valves. Furthermore, the rate of injection can be modulated by varying the voltage applied to the piezo-electric actuator.
Precise fuel injection is often difficult to achieve because many fuel injectors' performance is dependent on the speed and load of the engine. Secondly, many fuel injection devices rely on the pressure within a common rail which has a relatively large volume and provides a relatively constant pressure. A common rail, due to its relatively large volume, is relative slow to respond to any desired changes of pressure for sequential cycles of the fuel injector.
On the other hand, fuel injectors which use a cam or rocker arm to actuate a fuel injection piston in an individual injector have the fuel pressure and rate of injection quickly responsive to changes in conditions. However, the valve's injection rate of delivery is highly dependent on the engine speed. To eliminate the dependence on the engine speed, a fuel injector may have the opening and closing of an independent pressure control actuator controlled by a solenoid or a more responsive piezo-electric device connected to an electronic EUI controller.
One way to lower NOx emissions and meet stricter emission requirements is to regulate the rate of fuel injection and/or multiple injections such as a controlled pilot injection followed by a main injection. Previous injectors allow a pilot injection but only at the expense of the initial pressure and the fuel injection rate and quantity of the main injection. In addition, the injection rate with respect to time is naturally sloped having a relatively linear build-up from a low initial rate to a high rate near the end of injection. Attempts have been made to contour the injection rates by using injector cams with high velocity profiles but these specialized cams may provide only limited effect due to mechanical design constraints. It has also been proposed to provide a post injection after main injection, to reduce particulate emissions.
What is needed is a fuel injector that provides highly controlled fuel injection rates independent of engine speed and also responsive enough to provide a pilot injection phase and a post injection phase without degradation of the main injection phase.